Growth hormone (GH) and cortisol responses to perturbation of the hypothalamic-pituitary-somatomedin axis have been widely used in psychiatric research as a peripheral correlate of central noradrenergic activity. "Blunted" GH-responses are generally attributed to post-synaptic noradrenergic receptor down-regulation. We have administered clonidine to healthy controls, and to patients with social phobia or panic disorder. In addition, growth hormone releasing factor (GRF) was administered to normal controls and panic disorder patients. Compared with normal controls, both patients with social phobia and panic disorder exhibited "blunted" GH responses whether examining log-transformed GH data or the proportion of patients with + GH responses (defined as delta GH >5 ng/ml). In addition, the panic disorder patients had blunted GH-response to GRF.